Apparatus and method for detecting surface defects on a workpiece such as a rolled/drawn metal bar

ABSTRACT

The present invention is directed to solving the problems associated with the detection of surface defects on metal bars as well as the problems associated with applying metal flat inspection systems to metal bars for non-destructive surface defects detection. A specially designed imaging system, which is comprised of a computing unit, line lights and high data rate line scan cameras, is developed for the aforementioned purpose. The target application is the metal bars (1) that have a circumference/cross-section-area ratio equal to or smaller than 4.25 when the cross section area is unity for the given shape, (2) whose cross-sections are round, oval, or in the shape of a polygon, and (3) are manufactured by mechanically cross-section reduction processes. The said metal can be steel, stainless steel, aluminum, copper, bronze, titanium, nickel, and so forth, and/or their alloys. The said metal bars can be at the temperature when they are being manufactured. A removable cassette includes various mirrors. A protection tube isolates the moving metal bar from the line light assembly and image acquisition camera. A contaminant reduction mechanism applies a vacuum to remove airborne contaminants.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of U.S. applicationSer. No. 11/194,985 filed on Aug. 2, 2005 now U.S. Pat. No. 7,324,681entitled “AN APPARATUS AND METHOD FOR DETECTING SURFACE DEFECTS ON AWORKPIECE SUCH AS A ROLLED/DRAWN BAR”, now allowed, which in turn is acontinuation-in-part (CIP) of U.S. application Ser. No. 10/331,050 filedon Dec. 27, 2002 entitled “APPARATUS AND METHOD FOR DETECTING SURFACEDEFECTS ON A WORKPIECE SUCH AS A ROLLED/DRAWN METAL BAR”, now U.S. Pat.No. 6,950,546, which in turn claims the benefit of U.S. ProvisionalApplication No. 60/430,549 filed Dec. 3, 2002, the disclosures of whichare each hereby incorporated herein by reference in their entirety.

This application is also a continuation under 35 U.S.C. § 120 and §§365(c) of PCT International Application No. PCT/2006/029884 filed Jul.31, 2006 entitled “AN APPARATUS AND METHOD FOR DETECTING SURFACE DEFECTSON A WORKPIECE SUCH AS A ROLLED/DRAWN METAL BAR”, now pending, which inturn claims the benefit of U.S. application Ser. No. 11/194,985 filed onAug. 2, 2005, the disclosures of which are each incorporated byreference in their entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention or portions thereof was made with United Statesgovernment support under Cooperative Agreement No. 70NANBOH3014 awardedby the National Institute of Standards and Technology (NIST). The UnitedStates government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Related Field

The present invention relates generally to an imaging system that canimage the surface details of a workpiece, such as a rolled/drawn metalbar.

2. Description of the Related Art

It is known to produce a metal bar by a mechanical process such asrolling or drawing. Such metal bar is different than a metal slab,bloom, or strip (hereafter referenced as Metal Flat) in that the crosssection of such a bar has a smaller circumference/cross-section-arearatio such that the bar may rotate/twist about a longitudinal axis whilemoving forward longitudinally. For example, the bar shapes shown in FIG.2 have a ratio of circumference to cross-sectional-area that is equal toor smaller than 4.25 when the cross sectional area is unity for thegiven shape. The shape, when taken in cross section, of such a metal barmay be a round shape (item 102), an oval shape (item 104), or apolygonal shape, as shown as a hexagon (item 106), octagon (item 108) ora square (item 110) in FIG. 2. Furthermore, such a metal bar issubstantial in length. The length to circumference ratio is typicallyover 10 and the length to cross-section critical dimension (such as thediameter of a round bar or the side width a square bar) is over 30. Ametal bar of this type is typically referred to as “long products”rather than “flat products” in the related industries. Rolling, drawing,extrusion and the like, as used in this disclosure and hereafterreferenced as a Reducing Process, describe the ways for reducing thecross sectional dimensions of the metal workpiece through mechanicalcontact of the applicable tools, such as rollers and drawing dies, andthe workpiece. These Reducing Processes are generally continuous, orsubstantially continuous, in nature.

In the metal production industry, the presence or absence of surfacedefects is a relevant criterion upon which assessments of the metalproducts are made. For instance, surface defects account for half of theexternal rejects (i.e., rejected by the customer) for the steel bar androd industry. However, the conventional art provides no reliable meansto detect such defects. There are several challenges that conventionalinspection approaches have been unable to overcome.

First, in the case where inspection occurs while the metal bar productsare “hot,” the temperature can be as high as 1,100° C., preventing theuse of many inspection technologies. Second, the traveling speed of sucha metal bar along its longitudinal axis as described above can be,presently, as fast as 100 m/s, several times faster than the speed ofthe fastest metal strip and nearly 100 times faster than a metal slab orbloom. Further, speed increases are expected in the near future in therange of 150 m/s to 200 m/s. Conventional inspection approaches simplycannot accommodate such high traveling speeds. Third, a high temperaturemetal bar such as described above is typically confined in a sectionalconduit so that the bar will not cobble. Cobbling is an incident whereina hot, high speed metal bar runs freely outside the conduit. The space,therefore, for any inspection device is extremely limited. Last, thelength of such a metal bar, together with the fact of its longitudinalmotion, makes the handling of the bar difficult and costly.

While it is known to apply various imaging approaches to the inspectionof cast or rolled Metal Flats in line, visible light imagingtechnologies have heretofore not been used in in-line Long Products(i.e., metal bar with a substantial length) inspection. Conventionalimaging systems are not believed capable for use in inspecting metalbars and the like because the geometry of the metal bars invalidate theillumination and imaging designs that are used to enhance/capturedefects on flat surfaces. FIG. 4 illustrates the differences of applyingillumination and of capturing images on a flat workpiece (i.e., imageline 318 converges on illumination line on flat 316) versus a roundworkpiece. As to the non-flat workpiece, the freedom in opticalalignment and optical working ranges disappears when the object ofinterest does not have a flat surface. For instance, the image line 18and the illumination line 18′ may not overlap if the light or the camerais tilted, as shown in exemplary fashion in FIG. 4. One prior artapproach employs the use of area cameras to inspect the bar surfaces.However, it requires that the bar be stationary during imaging. Anotherprior art approach employs the use of line scan cameras, yet requiringthe bar to spin for the scanning due to its flat lighting design. Inorder to cope with the high longitudinal traveling speed,photo-sensitive diodes, instead of imaging sensors, are used in yetanother prior art. The use of photo-sensitive diodes limits thecapability of detection to short, transverse defects on the bar surface.This approach is incapable of detecting long, thin defects such as seamson a steel bar.

To avoid the lighting issue, use of infrared (IR) imaging devices isreported. In this approach, IR cameras are used to capture theself-radiated light from the long products. This approach is limited tothe surface defect detection solely based on surface temperature. It isknown that surface voids of a hot object appear to be hotter than theirneighborhoods due to the cavity theory, even though these voids are atthe same temperature as their neighborhoods. This approach is furtherlimited to its detection capability because of the focusing resolutionlimit of IR radiation. It is known to those skilled in the art that theoptical focusing resolution is inversely proportional to the wavelengthof the radiation. Typically IR cameras are nearly 10 times moreexpensive than a visible one and IR cameras are limited in their imagingspeed due to the sensor property. As a result, this approach would notbe able to accommodate the speed of today's long products.

Temperature also makes the long products different to their flatcounterpart. Metal bars typically are at a higher temperature than MetalFlats. Heat dissipation of an object is proportional to the area exposedto the cooling media, such as ambient air or water spray. The area of aMetal Flat is several times larger than that of a metal bar, assumingboth the flat and the bar are made of the same material and both havethe same longitudinal unit density and cross section area.

It is, however, known to employ imaging-based instruments for bar gaugemeasurement/control (shadow measurement), bar existence/presence, andbar traveling speed measurement in the Reducing Process.

It is also known to employ electromagnetic devices, such as eddycurrent-based instruments, in the assessment of long products.Eddy-current based sensing systems are used for the detection of surfaceimperfections in the Reducing Process for in-line inspection. Thisapproach has a high response rate, able to work in a high throughputproduction line environment (e.g., one kilometer of hot steel bars perminute). However, this approach has several drawbacks. First, it must bevery close to the hot surface (typically less than 2.5 mm). Accordingly,it is vibration sensitive and temperature sensitive. Moreover, it is notquantitative in the sense that it is NOT able to describe the nature ofthe detected defect. Finally, eddy-current approaches are incapable ofdetecting certain types of defects. As a result, the inspection outcomefrom eddy current devices is not used by the metal industry for adeterministic judgment on the quality of a specific product. Rather, theoutput of eddy current-based instruments is only used for qualitativeanalysis, such as “this batch of steel bars is generally worse than thebatch produced last week,” in the Reducing Process for process controlpurposes, for example, only.

Another approach attempted in the art employs ultrasonic sensing. Thisis an approach to replace the eddy current sensors with ultrasonic ones.However, many of the restrictions associated with eddy current-basedinstruments, such as the short working distance, apply with equal force.

Other inspection technologies used in the art include magneticpenetrant, circumflux, and infrared imaging with induction heating. Theuse of these technologies, however, is restricted. First, thesetechniques can only be used on “cold” metal bars. That is, thesetechnologies cannot be used for in-line inspection during or shortlyafter hot rolling applications. Also, the metal bars must be descaledbefore inspection. In addition, the use of magnetic penetrant is messyand cumbersome. This process typically relies on human observation withultra violet illumination, instead of automatic imaging and detection.The circumflux device is an eddy-current based unit, designed with arotating detection head. Such rotating mechanism limits the applicationof this device in metal bar inspection with high traveling speeds,typically used at about 3 m/s. Such device is also expensive due to themoving sensing head design. The combination of induction heating andinfrared imaging is based on the fact that induction current is onlyformed on the surface of the metal bar, and the surface defects on themetal bar will result in higher electrical resistance. Therefore, thespots with surface defects will heat up faster than other areas. Thereare issues associated with this approach in that (a) such faster heat upis a transient effect and thus timing (time to take images) is verycritical; and (b) infrared sensors are not available for very high datarates and therefore cannot support metal bars with high traveling speed.

Of course, inspection is possible after manufacture of the metal bars.However, post-manufacturing inspection often is not possible because theproduct is so long and coiled up, making the bar surfaces not accessiblefor cold inspection technologies.

Currently, real-time inspection of metal bars manufactured with ReducingProcesses is very limited. Metal bars are generally shipped from themanufacturer to the customer even if defective signals are posted by aconventional in-line eddy current inspection system. Customer complaintsmay therefore appear 3 to 6 months later due to surface defects on themetal bar products that are not immediately apparent to the customer.Such complaints cost the metal bar suppliers (i.e., manufacturers). Themetal bar suppliers will either refund the customers for the entirecoil/batch or cost share the expenses of additional labor to inspect theparts made out of the metal bar coil/batch.

There is therefore a need for an apparatus and method to minimize oreliminate one or more of the problems set forth above.

SUMMARY OF THE INVENTION

It is one object of the present invention to overcome one or more of theaforementioned problems associated with conventional approaches for animaging-based apparatus suitable to be used in-line or off-line todetect surface defects on rolled/drawn metal bars.

The present invention is directed to solving one or more of the problemsassociated with conventional metal bar inspection systems as well asproblems associated with applying metal flat inspection systems to metalbars for non-destructive inspection of surface defects on metal barsthrough the use of an imaging system.

One advantage of the present invention is that it may be effectivelyemployed in the production of metal bars with the aforementionedcharacteristics, namely, those that may be at a manufacturingtemperature, perhaps even hot enough to produce self-emitted radiation,as well as those subject to rotation relative to a longitudinal axis andmay potentially be traveling at a very high speed. Others advantages ofthe present invention include (i) effectively employed to image anddetect defects on non-flat surfaces; (ii) use for inspecting metal barsregardless of its temperature; (iii) use for inspecting metal barstraveling at speeds at or faster than 100 m/s; (iv) providing anincreased working distance to the metal bar surface, thus minimizing oreliminating the problems set forth in the Background for eddy-currentbased instruments; (v) providing an output comprising quantitative datawith verifiable defective site images; (vi) inspection of the workpieceeven before the scale forms on its surface; (vii) suitable for use ininspection at any stage (between the reducing stands or at the end ofthe line) in the reducing process, not affected by or relying upontransient effects; (viii) providing real-time or near real-time surfacequality information; (ix) providing a system absent of any movingsensing heads, thus minimizing or eliminating the problems of movingcomponents set forth in the Background; (x) providing a system needingonly very small gap (less than 50 mm) capable of operating between metalbar guiding conduit sections; and (xi) requiring no additional barhandling mechanisms/apparatuses However, an apparatus and/or method neednot have every one of the foregoing advantages, or even a majority ofthem. The invention is limited only by the appended claims.

A system is provided for imaging an elongate bar extending along alongitudinal axis. The system includes an image acquisition assembly, aline light assembly, and a computing unit. The image acquisitionassembly has a field of view configured to image a first predeterminedwidth over a circumference of a surface of the bar to define an imagebelt. The image acquisition assembly is further configured to produceimage data corresponding to the acquired image belt.

The line light assembly is configured to project a light line belthaving a second predetermined width onto the surface of the bar. Thelight line assembly is disposed, for example by alignment, relative tothe image acquisition assembly such that the image belt is within thelight line belt. The light line assembly is further configured such thata light intensity is substantially uniform along the image belt when thelight is collected by each of the image acquisition sensors.

For packaging purposes, the line light assembly may include a collectionof reflecting elements such as mirrors to achieve the designedprojection angle. For serviceability, the collection of the reflectingelements is designed to be detachable.

Finally, the computing unit is coupled to the image acquisition assemblyand is configured to receive image data for a plurality of image beltsacquired by the image acquisition assembly as the bar moves along thelongitudinal axis. The computing unit is further configured to processthe image data to detect predetermined surface features of the bar. In apreferred embodiment, the detected features are surface defects and theimage acquisition assembly includes n digital cameras, where n is aninteger 3 or greater, arranged so that a combined field of view of thecameras corresponds to the image belt.

A method of imaging a metal bar is also presented.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, wherein like reference numeralsidentify identical components in the several figures, in which:

FIG. 1 is a schematic and block diagram view of an embodiment of thepresent invention.

FIG. 2 are cross-sectional views of exemplary geometries for work piecessuitable for inspection by an embodiment according to the presentinvention.

FIG. 3 illustrates a cross-sectional geometry of a metal flat.

FIG. 4 is a diagrammatic view illustrating a conventional lightingscheme as applied to a metal flat and a bar.

FIG. 5 are simplified perspective views illustrating a bar constrainedduring its travel by a conduit, and a gap between adjacent conduits inwhich an embodiment according to the invention may be situated.

FIG. 6 is a simplified plan view illustrating an imaging coverage on ametal bar using one camera.

FIG. 7 is a simplified plan view illustrating an imaging coverage on ametal bar with one camera and a telecentric lens.

FIG. 8 is a simplified plan view illustrating an arc length variationbased on a projection of same size grids, such as a line of pixels, ontoa bar profile.

FIG. 9 is a simplified plan view illustrating a lighting arrangement fora bar surface in accordance with the present invention.

FIG. 10 is a simplified plan view further illustrating, in greaterdetail, the lighting arrangement of FIG. 9.

FIG. 11 is a simplified perspective view of a metal bar in connectionwith which the lighting arrangement of the present invention is used.

FIG. 12 is a simplified plan view illustrating the lighting arrangementin the circumferential direction as directed toward a bar surface.

FIG. 13A is a plan view illustrating another embodiment of a lightingand imaging arrangement for a bar surface according to the invention.

FIG. 13B is a diagrammatic view of the lighting arrangement of FIG. 13Asuch that the collection of reflective elements can be retrieved forcleaning and restored for function easily, as shown in an installedposition.

FIG. 13C is a diagrammatic view of the lighting arrangement of FIG. 13B,including the collection of reflective elements, shown in a partiallyremoved position.

FIG. 13D is a diagrammatic, perspective view of the embodiment of FIG.13B, showing a protective tube.

FIG. 13E is a diagrammatic, side view of the embodiment of FIG. 13B.

FIG. 13F is a diagrammatic, perspective view of the embodiment of FIG.13B viewed from an opposite side relative to that in FIG. 13D.

FIG. 14A illustrates a surface defect along with some surface noise.

FIG. 14B illustrates an exemplary result of an image processing stepaccording to the invention as applied to the image of FIG. 14A.

FIGS. 15A-15C illustrate examples of long surface defects that may befound on metal bars and which can be detected by an embodiment accordingto the present invention.

FIGS. 16A-16C illustrate relatively short surface defects that may befound on metal bars and which can be detected by an embodiment accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention permits automated inspection of metal bars forsurface defects as the metal bars are being rolled, drawn or the like(i.e., the reducing process described in the Background of theInvention) without additional handling to the bars. FIG. 1 schematicallyillustrates a preferred embodiment in accordance with the presentinvention.

Before proceeding to a detailed description of the present inventionkeyed to the drawings, a general overview will be set forth. The presentinvention provides the following features:

1. Capable of working for metal bars manufactured through reducingprocesses at different cross section geometry;

2. Capable of working for metal bars in-line at a bar temperature up to1,650° C.;

3. Capable of working for metal bars traveling at 100 m/s or higher;

4. Capable of detecting surface defects whose critical dimensions are assmall as 0.025 mm;

5. Capable of reporting the defect nature such as its size, location (onthe bar), image, and the like;

6. Capable of accommodating different sizes of bars, for example only,from 5 mm to 250 mm with minimum adjustment;

7. Capable of providing real-time or near real-time inspection results;

8. Capable of working with a small access window (less than 50 mm) tothe target object;

9. No moving parts while inspecting;

10. No additional handling of the bars; and

11. Continuous operation in commercial, heavy industrial metalproduction mills.

FIG. 1 is a simplified schematic and block diagram of a system inaccordance with the present invention. FIG. 1 shows line light assemblywhich may include at least one light source 2, a light conduit 4, aplurality of line lights 6 and a corresponding plurality of opticalboosters 8. FIG. 1 further shows a computing unit 10 and an imageacquisition assembly that may include a plurality of cameras 12 eachhaving a corresponding lens 14.

With continued reference to FIG. 1, a workpiece or object underinspection, such as an elongated metal bar 16 extending along alongitudinal axis, is shown moving along its longitudinal direction 20at a speed up to 100 m/s or faster while bar 16 is going through areducing process. The metal bar 16 may be formed from one selected fromthe group comprising steel, stainless steel, aluminum, titanium, nickel,copper, bronze, or any other metal, and/or their alloys. The bar 16 maybe solid or hollow. Typically such metal bar 16 is traveling inside aconduit, as shown in greater detail as conduit 24 in FIG. 5, not shownin FIG. 1. A gap 26, shown in FIG. 5, is defined between two adjacentconduits 24, and is typically very small, for example between about 20to 50 mm taken in the axial direction for high-speed transit of metalbars 16. It should be understood that metal bar 16 may be at an elevatedtemperature, for example as hot as 1,100° C. for a hot rolling process.It should also be appreciated that metal bar 16, given its geometry, isprone to twist/rotate about its longitudinal axis uncontrollably in thedirection indicated by arrow 21 in FIG. 1 when it travels in direction20. This possibility for uncontrollable rotation has, among other items,presented problems for conventional imaging systems. As will bedescribed in greater detail below, the present invention overcomes thisproblem to provide an imaging system that is robust to twisting and/orrotation.

In order to detect surface defects on bar 16, an imaging system inaccordance with the present invention must be provided having certainfeatures, as described below. With continued reference to FIG. 1, theimaging system includes an image acquisition assembly that preferablycomprises n imaging cameras 12, wherein n is an integer 3 or greater.The parameter n is selected to be three or higher based on an analysisset forth below. Each camera 12 is arranged so as to cover acircumferential span of at least 120° in order to image the entiresurface of bar 16. That is, the image acquisition assembly has acomposite or combined field of view configured to image the entirecircumference of the surface of the bar 16 to define an image belt 18.As further described below, the image acquisition assembly is furtherconfigured to produce image data based on the image belt 18. Theanalysis for the parameter n for the number of cameras will now be setforth.

As shown in FIG. 6, a regular lens 14 associated with camera 12 willhave a viewing angle (field of view) formed by the two tangential linesof sight 28 extending from a focal point of lens 14 to the surface ofbar 16. This viewing angle, when projected onto a non-flat surface, suchas the one shown in FIG. 6, will result in a circumferential coverage 30that is less than 180° and will be insufficient to cover 360° with onlytwo lens/camera units where the lens are not telecentric.

FIG. 7 shows an arrangement with a telecentric lens 14′. A truetelecentric lens, which collects lines of sight that are in parallel,even if used, would not practically provide for a two-lens/camera systembecause of arc length variation. In particular, the lines of sight 28are parallel with the addition of telecentric lens 14′ to lens 14. Inthis case, the circumferential coverage 30 is 360°. Theoreticallyspeaking, the entire surface of round shaped bar 16 can be covered usingonly two lens/camera units. However, as alluded to above, a problem ofnon-uniform pixel sizes arises.

As illustrated in FIG. 8, the evenly spaced lines of sight 34, asderived from an evenly-spaced imaging sensor 32 having a plurality ofpixels, can result in an uneven arc length 36 on the surface of bar 16,pixel-to-pixel. Even spacing is a very typical arrangement on an imagingsensor such as a CCD chip. The arc length 36 can be calculated usingequation (1) as follows:S=p/cos(θ)  EQN (1)

where S is the arc length 36 mapped to the pixel in position y, p is thepitch of the pixel array or the pixel size, and θ is the projected anglethat can be derived fromθ=arcsin(y/r), in which y≦r and r is the radius of the metal bar 16.  EQN (2)

From FIG. 8 one can learn that as y→r, θ→90°. As θ→90°, S the arc length36 will approach infinity based on EQN (1). In reality, S will still bea finite number. However, S will be substantially (several times) largerthan p, the pixel size. That is, the image resolution in this area willdeteriorate so much that this approach is infeasible. Note that the samearc length analysis can be applied to the bottom half in FIG. 8, inwhich case y→−r.

With three cameras, θ can be established at 60°. When θ=60°, S the arclength 36 (at the 12 o'clock and 6 o'clock positions in FIG. 8) is only2 p, an acceptable and controllable deterioration in image resolution.If better image resolution is desired, four cameras or five cameras, oreven more may be used (i.e., the parameter n referred to above can be aninteger equal to four, five or greater). All the lens 14/camera 12combinations, as illustrated in FIG. 1, are preferably arranged suchthat all such lens/camera combinations are positioned along a circularpath 22 that is concentric to the circular geometry of the exemplarymetal bar 16 such that the working distances, the distance from eachlens 14 to the nearest metal surface, are the same or nearly the samefor all the lens/camera combinations. Note that the path 22 may staycircular if the metal bar is non-circular, say a hexagon, for thepurpose of generally serving the same manufacturing line. One that isskilled in the art can appreciate that the path 22 can, if desired, bemade to conform to the actual bar geometry.

In order to accommodate the potentially very high traveling speed of themetal bar 16, high data rate cameras 12 are preferably used. The cameras12 in the system are thus preferably digital cameras, with digitaloutputs to computing unit 10. This digital output format is desirable toaccommodate the harsh environment for signal fidelity. This digitalformat image signal may be received by the computing unit 10 throughstandard communication channels such as IEEE-1394 (also known asFireWire), Camera Link or USB ports, or a special interface known as aframe grabber. Each camera 12 preferably is able to generate at least10,000,000 (or 10 Mega) pixels per second such that a defect featurethat is 0.025 mm×0.5 mm can be identified. It should be appreciated,however, that to detect larger features, a reduced resolution, and hencereduced data rate (in pixels per second) would be required. Line scancameras are preferred even though progressive (non-interlaced) area scancameras can be used when the bar 16 is not traveling fast. Line scancameras have an advantage over area scan cameras in that line scancameras only require a line of illumination, instead of an area ofillumination. This will simplify the illumination complexity caused bythe non-flat surface. In the case of using line scans, all the camerasin FIG. 1 will be aligned such that their imaging lines will be forminga circumferential ring, an image belt 18, on bar 16. This alignment isnecessary to address the issue of twist and/or rotation (item 21). Ifthis alignment is not held, the twisting or rotating motion can resultin incomplete coverage of the bar surface.

Referencing back to FIG. 1 again, each camera will have a lens 14 tocollect light reflected from the bar surface. Telecentric lenses (lensesthat collect parallel image light rays, as illustrated with FIG. 7) arepreferred for a more uniform arc length distribution, even thoughregular lenses can be used. In addition, cameras 12 may be configured toinclude a lens iris to control exposure, and further, preferablyconfigured (if included) with the use of a predetermined lens irissetting for improved depth of focus/field in the application.

With continued reference to FIG. 1, the imaging system according to thepresent invention also includes a line light assembly configured toproject a light line belt onto the surface of the metal bar 16.Preferably, the line light assembly includes a plurality of line lights6. These line lights 6 can be individual light sources, such as lasers,or light delivery devices, such as optical fiber lights, as shown inFIG. 1. The light delivery devices must work with at least one lightsource, as shown in FIG. 1. More than one light source can be used ifhigher light density is needed for illumination. For metal bars 16 thattravel at very high speed, the cameras may be light starved due to veryhigh line/frame rates equating to a relatively short exposure time. Anoptical booster 8 may therefore be used for each line light toconcentrate the light and increase the light intensity. This booster 8can be a cylindrical lens or a semi-cylindrical lens. To use the imagingsystem in accordance with the present invention for metal bars 16 thatare at an elevated temperature, the line lights and the boosters must bemade of special materials configured to withstand such elevatedtemperatures. Each line light 6, for example, may be configured to haveits own glass window to serve this purpose. In the case of optical fiberline light, the material that binds the fibers together must be able towithstand high temperature, such as the high temperature epoxy. Theboosters 8 must be made of materials that can withstand hightemperature, too. Usable materials include glass, Pyrex, crystal,sapphire, and the like.

FIG. 9 is a top view of the preferred embodiment shown in FIG. 1. Tocope with light starving, the alignment between the line lights and thecameras is important. As illustrated in FIG. 9, the surface of metal bar16 after the reducing process, before, for example, a descaling process,can be treated as a reflective surface. Therefore, the optical law setforth in equation (3) applies:“incident angle=reflective angle”  EQN (3)

EQN (3) is preferably used in a preferred embodiment to maximize thereflected light that is captured by the plurality of cameras 12. Theline lights 6 will each emit the light ray 40, which is boosted by abooster 8 and projected onto the surface of the metal bar 16. The lightray 40 is reflected to the path 42 and received by the lens 14 andeventually by the camera 12. Note that in FIG. 9, the metal bar 16travels in the direction 20. The projected and reflected light rays 40and 42 form an angle 44, equally divided by the normal line to thesurface of the metal bar 16. This angle 44 must be as small as possible,due to the illumination problem described above that is associated witha non-flat surface, as illustrated in FIG. 4. In FIG. 4, the light line18′ and the image line 18 will not overlap on a non-flat surface. Theideal case is for the angle 44 in FIG. 9 to be 0°. As this is onlypossible by using a beam splitter, it is less practical to do so whenthe system is light starving due to inherent power losses imposed byusing a beam splitter for example. The highest efficiency a beamsplitter can achieve is 25%, assuming a 0% transmission loss. Therefore,the angle 44 is preferably selected so as to be reasonably small, suchas 1° or in its neighborhood. If necessary, a reflective mirror 38 canbe used to assist in packing the camera and the light for a small angle44. This is another reason to use line scan cameras in this application.Line scan cameras only need an image path 42 with a small width, such asfrom 5 to 30 microns. The angle 44 can be kept very small with thissmall image path feature.

FIG. 10 shows in greater detail a portion of the lighting setup of FIG.9. As mentioned above, the angle 44 will not be 0 degrees unless a beamsplitter is used. Therefore, each line light 6 must have a substantialwidth W (item 41 in FIG. 10). One can see that in FIG. 10 the metal bar16 has a centerline 46. The line 48 indicates the 60° mark on the barsurface, starting from the tangential boundary on the left hand side ofthe bar, as shown in FIG. 10, and increasing to the right. One cameramust be able to image the metal bar 16 for the upper half to this 60°mark line 48. In a three-camera embodiment, the above calculationsapply. If more cameras are used, the line 48 may represent 45° for afour-camera system, at 36° for a five-camera system, and so forth. Ifdesigned symmetrically, the camera can also image the bottom half of themetal bar 16 for 60°. In order to achieve this coverage, the light linewidth W must be greater than a threshold based on:W≧2·r·(1−cos 60°)·sin α  EQN (4)

in which r is the bar radius and α is the incident angle (half of theangle 44). The 60° can be replaced by another angle if a differentnumbers of cameras other than three are used in the inventive imagingsystem. This notion is further illustrated in FIG. 11, in which theimage line 42 is clearly curved differently, yet covered by the lightline 40. In other words, the image acquisition assembly (e.g., theplurality of cameras in the preferred embodiment) captures an image belt18 having a first predetermined width over the entire circumference ofthe surface of the bar 16. The line light assembly (e.g., the pluralityof line light sources in the preferred embodiment) projects a light linebelt onto the surface of the bar 16 having a second predetermined width.The line light assembly is disposed and aligned relative to the imageacquisition assembly such the image belt falls within the light linebelt. Through the foregoing, the problem of non-flat surfaces isovercome.

Additionally, these line lights must be positioned such that the lightintensity as reflected from a point on the bar surface to the camerathat covers that point is uniform for all the points on the image belt18 (FIG. 1). A more detailed illustration is shown in FIG. 12. All theillumination must follow the law described in EQN (3). FIG. 12illustrates this arrangement for one camera. It should be appreciatethat such arrangement may be duplicated for other cameras used in theinventive imaging system. Based on EQN (3), the angle formed by theincident light ray 40 and the reflected light ray 42 must be evenlydivided by the surface normal 50. As in FIG. 12, an illuminator 52preferably includes a curved surface. Illuminator 52 is a device whoseemitted light rays (perpendicular to this curved surface at the point ofemission) will be reflected by the surface of the bar 16 to the imagingsensor in camera 12 and lens 14 based on EQN (3). Note that curve 52need not be a circular curve. This curve 52 depends on the distancebetween the curve 52 and the surface of the bar 16 (i.e., target). Curve52 may not be a smooth curve if the bar is not circular. Even though anilluminator with curve 52 can be made with modem technologies, suchilluminator can only be used with bars 16 at the designated diameter. Insome applications it is not practical. An alternative is to simulatesuch illumination effect with an array of light lines 6 and 8, as shownin FIG. 12. Each combination of light line/booster can be madeadjustable such that its direction can be re-pointed as shown by item 54to accommodate targets with different diameters. The light line approachis also beneficial in the case that the bar 16 is not circular.

FIG. 13A is a simplified schematic and block diagram view of anotherembodiment of a system in accordance with the present invention. Thisembodiment provides a very easily serviceable cassette containingreflective elements that are packaged in a relatively small space (e.g.,20 to 50 mm) so as to be operable in the small access gap 26 (best shownin FIG. 5) with the workpiece/moving bar 16 being contained andlongitudinally moving in direction 20 through conduit 24 or the like.FIG. 13A shows line light assembly 6, optical booster 8, camera 12, lens14, reflective mirror 38 for the incident/illuminated light ray 40, asecond reflective mirror 38′ for the reflected (image) light ray 42representing the image of the bar surface, and a protection device suchas a tube 43 having a first part 43 a and a second part 43 b spacedapart and offset from the first part 43 a along axis “A” to define anaccess space 43 c. The protection parts 43 a, 43 b are configured toprotect the relatively fragile imaging and illumination components fromthe heat, shock (e.g., contact) and other contamination (e.g.,particles) originating from moving bar 16, which may be at an elevatedtemperature (as described above). Parts 43 a and 43 b may becircumferential. Aperture 43 c may be configured in size and shape toallow entry/exit of illumination light rays 40 and reflected (image)light rays 42. Protection tube 43 may be formed of metal or otherdurable material suitable for segregating hot steel bar 16 from the restof the inventive system.

FIG. 13A further shows a contamination reduction mechanism, employing avacuum, which is configured to reduce the presence of airborne,relatively small size contaminants 62, such as mill scale powder or tinywater mist, that may be present in the space proximate or near theaccess space 43 c of the protection tube 43 (i.e., contaminants that areemanating from the moving metal bar in the interior of the protectiontube). This contaminant reduction mechanism may operate in combinationwith air knives or air wipes or the like in and around access 43 c,which are configured to block larger items from fully exiting accessspace 43 c. One advantage of the vacuum-based contaminant reductionmechanism is that it keeps the optical components relatively contaminantfree, or at a reduced contaminant level, thereby keeping them cleanerand improving optical performance (i.e., compared to dirty opticalcomponents). Additionally, this contamination reduction mechanism may beoperative to reduce airborne contaminants in the imaging path, which mayimprove visibility of the image acquisition means.

The contaminant reduction mechanism includes a vacuum end effector 64having a vacuum connector 66, a pipe or other vacuum conduit 66′ havinga first end configured to connect to the connector 66, and a source ofvacuum such as vacuum pump 72, which is coupled to the other end ofconduit 66′.

The vacuum end effector 64 is preferably in a ring shape, as shown inthe figures, and is formed about a ring axis and having a vacuum suctioninlet 70 circumscribing the space in and around the access space 43 c ofprotection tube 43. As installed, the ring axis of the end effector 64is substantially coincident with the longitudinal axis along which themetal bar moves. End effector 64 is of thin-wall construction and of agenerally closed geometry whose plurality of contiguous, outer thin-wallsides define an interior vacuum chamber 68. In the preferred embodiment,the ring shaped vacuum end effector 64 is characterized by a rectangularshape in radial cross-section (best shown in FIG. 13E). It should beunderstood, however, that the vacuum end effector 64 may take variousshapes, for example, a pair of half-rings arranged in cooperatingfashion to provide vacuum coverage around the perimeter of access space43 c. Additionally, vacuum end effector 64 may take the form of aplurality of straight bars arranged around the perimeter of access space43 c. Other variations are possible, which are within the spirit and thescope of the present invention.

The vacuum source (pump 72) operates through the conduit 66′, vacuumchamber 68, and finally via suction inlet 70 to apply vacuum (and thussubstantially evacuate) the space proximate the access space 43 c,including any small, airborne contaminants 62.

FIG. 13B is a diagrammatic front perspective view showing a plurality ofreflective mirrors 38 (illumination directing) configured in a removablecassette 152. FIG. 13B shows eight reflective mirrors 38 supported by acorresponding number of mirror seats 138. Cassette 152 is removable andis shown in the installed position (FIG. 13B) and in a nearly, fullyremoved position (best shown in FIG. 13C). Cassette 152 is shown mountedonto a holder such as a plate 150. Plate 150 can be linked to otherelements of the inventive imaging system through a base plate 154.

In the illustrative embodiment, plate 150 is configured to include asliding groove 156 around an inner perimeter of plate 150. Cassette 152includes a plurality of fitting tabs 158 (four shown arranged indiametrically opposed pairs) configured in size and shape to mate withgroove 156. The dimensional tolerance is such that cassette 152,particularly the mirrors 38 thereof, will be properly aligned withcomponents 14/12 and components 6/8 when cassette 152 is in theinstalled position. It should be appreciated that cassette 152 includesthe passive components, namely, illumination directing mirrors 38 andimage reflecting mirrors 38′ (best shown in FIGS. 13E and 13F), and thusdoes not require any connections via cables, power wires or the like toother elements external to cassette 152 that comprise the inventiveillumination and imaging system. This provides an advantage inasmuch asthe cassette 152 may be removed for cleaning and reinstalled relativelyeasily due to the absence of such connections.

Cassette 152 may be maintained in the installed position (i.e., inalignment) through the use of a suitable locking and retentionmechanism, such as a simple closure member 153 (shown in phantom line inFIG. 13B), having suitable mating features to also slide in groove 156,and be retained therein (e.g., fasteners). One of ordinary skill in theart will appreciate that there are a wide variety of alternate suitablelocking and retention mechanisms.

In the embodiment of FIG. 13B, four cameras 12 are used.

FIG. 13B further shows the ring embodiment of the contaminant reductionmechanism in perspective view.

FIG. 13C is a simplified diagrammatic view of cassette 152 in theremoved position. Cassette 152 can be easily removed by firstdefeating/disabling any locking and retention mechanism 152 that may bein-use, and removing the cassette in the direction of arrow 160, forexample, for servicing (e.g., cleaning, repair, re-alignment). Thecassette 152 may be remounted/reinstalled easily by reversing theabove-described procedure.

FIG. 13D is a diagrammatic, front perspective view of the embodiment ofFIG. 13B showing a tube-shaped protection device 43 a and 43 b. FIG. 13Dalso shows locking and retention mechanism 153 in the installed, lockedposition. In the installed position as shown, the illumination directingmirrors 38 and the image directing mirrors 38′ are in alignment with thelight line assembly (light source 6 and booster 8) and lens 14/camera12, respectively.

FIG. 13D further shows vacuum end effector 64, as taken in radialcross-section through the middle (half of the ring omitted for clarity).As shown, the end effector 64 includes a plurality of thin-wall sidesdefining the interior vacuum chamber 68. Suction inlet 70 is also shown,which may be formed by removing a radially inwardly facing corner toreveal the suction inlet 70.

FIG. 13E is a diagrammatic, side view of the embodiment of FIG. 13B,with the cassette 152 in the installed position. FIG. 13E shows imagedirecting mirrors 38′ in alignment with lens 14 and camera 12. FIG. 13Ealso shows the camera viewing gap 43 c defined in between protectiontube portions 43 a and 43 b.

FIG. 13E further shows a side view of the cross-sectioned vacuum endeffector 64 of FIG. 13D. FIG. 13E shows as a side plan view the interiorvacuum chamber 68 and the vacuum suction inlet 70. Inlet 70 facesgenerally radially inwardly toward, as well as circumscribing theperimeter of the access space 43 c.

Vacuum end effector 64 may be formed using conventional constructiontechniques and materials (e.g., metal or other durable materials).Vacuum connector 66 and conduit 66′ may also comprise conventionalconstruction techniques and materials known to those of ordinary skillin the art. Additionally, the vacuum pump 72 may also compriseconventional apparatus known to those of ordinary skill in the art. Forexample, the vacuum pump 72 may be a venturi or electrical type or othertype known in the art.

The desired, preselected applied vacuum level (i.e., level of vacuum asobserved at suction inlet 70) may be determined by the particulardegree, presence and type of small contaminants 62, and the geometry of,orientation and proximity of suction inlet 70 with respect to accessspace 43 c. The corresponding performance characteristics of vacuum pump72 may be determined based on the above determined applied vacuum leveldesired, in view of the particular geometry and size/volume of endeffector 64, as well as the size and length of connector 66 and conduit66′.

FIG. 13F is a diagrammatic rear perspective view of the embodiment ofFIG. 13B. FIG. 13F shows three image directing mirrors 38′ in cassette152 (one mirror 38′ for each lens 14/camera 12 combination). Note, onemirror 38′ is obscured in FIG. 13F, as is the corresponding lens14/camera 12 combination.

Referencing back to FIG. 1, a computing unit 10 is coupled to pluralityof cameras 12. The computing unit 10 is configured to receive the imagedata for a plurality of image belts 18 acquired successively by thecameras 12 as the bar 16 moves along the longitudinal axis in thedirection 20 (direction 20 best shown in FIG. 1). Frame grabbers may beused to receive the image signals. The cameras 12 in the system,however, are preferably digital cameras, as described above. Thecomputing unit may comprise one or more computers in order to haveenough computing power to process the image data. Image processinghardware may be used in conjunction with the software for fastercomputing speed. If multiple computers are used, these computers can belinked together through inter-computer links such as TCP/IP or the like.

In any event, computing unit 10 is configured to process the image datato detect predetermined features of the surface of bar 16. In apreferred embodiment, the features are surface defects. Thus, the imagedata will be processed for defects, such defects being shown inexemplary fashion in FIGS. 14A-14B. The images typically contain boththe real defects (e.g., item 302) and noise, such as scratch marks(e.g., item 304). Image processing algorithms, implemented in computercodes such as C, C++, machine languages, and the like, or implemented inhardware logic, are used to filter out the noise, and to detect the truedefects, as shown in 306. The defects to be identified can be long andhave a large aspect ratio, as shown in FIGS. 15A-15C, where item 308 maybe 1000 mm long, and item 310 may indicate a width of 0.050 mm. Or, thedefects can be short and have a nearly 1-to-1 aspect ratio, as shown inFIGS. 16A-16C. These algorithms are known in the art, but will bedescribed generally. A first layer of processing may involve acomparison of local contrast in the image, such as by comparing a firstpredetermined threshold to the local contrast. A second layer ofprocessing may involve applying a second predetermined threshold todetect the nature of the defect such as size, location, length and widthand the like.

The preferred embodiment described and illustrated in connection withFIG. 1 will also have protection against dust, water, vibrations, andother damaging factors in a typical metal process plant such as a hotrolling mill or a cold drawing mill.

Those skilled in the art shall appreciate the possibility of furtherrestrain the bar and separately using three or more single-camerasystems in the reducing process line for inspection.

Those skilled in the art shall also appreciate that covering (e.g.,inspection of) a portion of the bar surface less than the entirecircumference may be useful enough for statistical process controlpurpose in the reducing process line.

Those skilled in the art shall also understand that a very high speed(high data rate and high frame rate) area scan camera can be used inplace of the line scan cameras if only a certain portion of each of thearea scan images is used for processing.

One can also understand that if the metal bars are at an elevatedtemperature, an optical filter can be used in conjunction with the lenssuch that only certain wavelengths in the reflected light rays 42 (inFIG. 12) will be used to carry the surface information of the metalbars. Such wavelengths are those not emitted or not dominantly emittedby the metal bars at the said elevated temperature. For metal bars at orcolder than 1,650° C., the wavelength 436 nm can be used. In this case,an interference filter at 436 nm will be used with the lens. Thiswavelength can vary with the temperature. If the temperature decreases,longer wavelength can be used.

In a still further variation, the light line assembly may be configuredto include a strobe light, wherein the computing unit 10 synchronizesthe illumination (i.e., the strobing) with the image capture functionperformed by the image acquisition assembly (e.g., the cameras 12 in thepreferred embodiment).

In a yet still further embodiment, the computing unit 10 is configuredto maintain a running record of the detected defects, including (i) arespective location of each detected defect relative to a “start”position, such as the leading end, on the bar 16 being manufacturedthrough processes that mechanically reduce the cross-sectional area ofthe metal bars; (ii) a respective notation of the nature of the detecteddefect, such as the size, shape, contrast; and (iii) optionally, anactual image of the site of and surrounding the detected defect. Therecord may be useful to the supplier/manufacturer, for example, fordetermining an up-front discount, and may be provided to the customer(e.g., on a diskette or other electronic means) for use in furtherprocessing, for example, what portions of the bar to avoid or dofollow-up work on.

1. A system for imaging an elongated bar extending and moving along alongitudinal axis in a manufacturing process, said system comprising: animage acquisition assembly having a field of view configured to image afirst predetermined width over a circumference of a surface of said barwhile said bar is moving to define an image belt and produce image datacorresponding thereto, said image acquisition assembly including ndigital cameras, where n is an integer 3 or greater, arranged so that acombined field of view thereof corresponds to said image belt, saidcameras comprising line scan cameras; a light line assembly configuredto project a light line belt having a second predetermined width ontothe surface of said bar, said light line assembly being disposedrelative to said image acquisition assembly such that said image belt iswithin said light line belt, said light line assembly being furtherconfigured such that a light intensity is substantially uniform alongsaid image belt; a protection device comprising a tube having a firstpart and a second part spaced apart and offset from said first partalong said longitudinal axis to define an access space, said tube beingdisposed intermediate the elongated bar and said image acquisition meansand said light line assembly, said access space being configured in sizeand shape to allow (i) entry of said light line belt and (ii) exit ofsaid image belt; a contaminant reduction mechanism configured to reducethe presence of contaminants in the space proximate said access space ofsaid protection tube; and a computing unit coupled to said imageacquisition assembly configured to receive image data for a plurality ofimage belts acquired by said image acquisition assembly as said barmoves along said longitudinal axis, said computing unit being furtherconfigured to process said image data to detect predetermined surfacefeatures of said bar.
 2. The system of claim 1 wherein said contaminantscomprise one of mill scale powder and water mist.
 3. The system of claim1 wherein said contaminant reduction mechanism comprises a vacuum endeffector having an outer wall defining an interior vacuum chamber, saidend effector further including a suction inlet located proximate saidaccess space of said protection device, said vacuum end effector beingconfigured to be connected to a vacuum source.
 4. The system of claim 3wherein said vacuum end effector is a ring shape having a ring axis thatis substantially coincident with said longitudinal axis, said suctioninlet being configured in size and shape to circumscribe the perimeterof said access space.
 5. The system of claim 4 wherein said vacuum endeffector is substantially rectangular in radial cross-section, saidsuction inlet being formed by removal of a radially-inward corner ofsaid vacuum end effector.
 6. The system of claim 3 wherein said vacuumend effector comprises a pair of half-ring shape body portions.
 7. Thesystem of claim 3 wherein said vacuum end effector comprises a pluralityof straight bars.
 8. The system of claim 3 wherein said contaminantreduction mechanism further includes a conduit for connecting saidsource of vacuum to said vacuum end effector, said source of vacuumcomprising a vacuum pump.
 9. The system of claim 8 wherein said vacuumpump is a venturi type.